I fail to see how that'* possible. When you add a 20+ gallon tank AND fill it you're significantly increasing the weight. If the car is still getting the same MPG how is it less efficient?
Also, it starts on gasoline to make sure the propane system is at the right markers. Since a normal ICE isn't deigned to run on propane, the propane has to be perfect. Right temp, be purged, and some other things I can't remember.
I think most of those figures are from the old style of propane fogger systems, they're not as effective as the new system which is DI.
2004 Chevy Silverado Z71 - 15% tint, FTDRL, All-on-high, 5000k lows/fogs, tinted tails, color matched grille, Sound by Kenwood.